In numerous applications in the electronic domain, current sources are required to drive electrical loads. For example, such current sources may be connectable in series with an electrical load, this series connection in turn being supplied by a voltage regulator. Here, the current source serves for precisely adjusting the current level for the electrical load.
Such precise current sources are used, for example, for driving lighting means, in particular light-emitting diodes (LEDs). For this purpose, the electrical load in the load branch to be connected to the voltage regulator may be connected in series with the controlled path of a transistor and with a resistor. In this arrangement, the transistor and the resistor are included in the current source.
Realizing the current source in integrated circuit technology is desired in many cases. Conventionally, a unipolar transistor such as a MOSFET (metal oxide semiconductor field effect transistor) is required for the power element to achieve the desired accuracy. The unipolar transistor is distinguished in that the level of the load current in the controlled path is adjusted by means of the potential on the gate electrode, which is thus possible in exact fashion. The drain and source currents are equal by definition.
This is why a bipolar transistor cannot be used there, because the base current does not allow an exact adjustment of the current on the collector, i.e. of the load current, when the transistor is interconnected in emitter configuration.